BACKGROUND

The term "geothermal" can be literally translated from Greek terminology to mean "from heat within the earth."

Geothermal power comes from the radioactive decay of minerals that helped to form planet Earth more than four billion years ago, among other factors.

Geothermal heat continuously flows from the earth's core towards the earth's surface. This results in the formation of hot springs, geysers, volcanoes, and
fumaroles (openings in the earth's crust) through faults and cracks in the surface.

Fluids from the earth are mostly water containing varying amounts of dissolved salts. These salts are present in the liquid phase, but sometimes consist of a saturated, liquid vapor mixture, often called "superheated steam vapor."

Geothermal power is responsible for hot mineral springs. The Chinese, Roman, and Native American (Indian) cultures used the springs for cooking and heating purposes. Some people believed that bathing in these waters produced natural medicinal benefits.

Other hydrothermal developments, including the steam field at The Geysers, California, and manmade hot water systems in the Philippines; Indonesia; Wairakei, New Zealand; and Cerro Prieto, Mexico, produce about 10,000 megawatts of electricity. More than 70 countries are currently reported to use geothermal power for direct use.

Electricity produced by geothermal power exists in more than 24 countries, some of which receive 15-22% of their national electricity production from it.

TECHNIQUE

District heating system: Hot water or steam from near the earth's surface is used to heat homes and other buildings. The heat or steam is distributed via a network of insulated pipes. Usually made up of feed and return lines, these pipes can be located underground or aboveground.

Geothermal power plants: Geothermal power plants use "hydrothermal" ("water" and "heat") resources to generate electricity. These resources include dry steam or hot water reached by drilling wells deep into the earth and then piped to the earth's surface. In general, geothermal power plants require water or steam that is 300-700 degrees Fahrenheit.

Dry steam plants: These plants route steam piped from geothermal reservoirs directly through turbines or generator units to produce electricity. This is the oldest type of geothermal power plant and was first used in 1904 in Lardarello, Italy.

Flash steam plants: These plants convert hot water from deep below the earth's surface into steam, then run through turbines or generator units to produce electricity. As the steam cools and condenses back into water, the water is injected back deep below the earth's surface to be recycled.

Binary power plants: These plants transfer heat from geothermal hot water to another liquid such as isopentane, which converts to steam and drives turbines or generator units to produce electricity.

Different types of geothermal heat pumps are used, depending on climate, soil conditions, available land, and local installation costs at the building site.

Open-loop system: This system uses well or a surface body of water as the heat-exchange fluid that circulates directly through the geothermal heat pump system. Once it has completed circulating through the system, the water returns to the ground through the well or surface body of water.

Closed-loop system: There are three types of closed-loops systems: horizontal, vertical, and pond/lake systems. A horizontal closed-loop system typically requires two pipes placed side by side at five feet in the ground in a two-foot-wide trench. They are then connected to the heating pump. A vertical closed-loop system requires holes drilled 20 feet apart and 100-400 feet deep. Two pipes are connected to these holes to form a loop. These loops are connected to another pipe, placed in trenches, and then connected to the building heating pump. A pond/lake closed-loop system uses a small body of water alongside the building. It involves four-foot-deep trenches and two pipes placed side by side at five feet in the ground in a two-foot-wide trench. The piping is looped and is connected to the heating pump.

Often, pipelines are mounted above the surface of the ground, so as not to interfere with farming, pasture, or other land use.

Water management: Water use in geothermal power plants is about five gallons of water per megawatt hour. Geothermal power plants are much more energy efficient than natural gas facilities, which use 361 gallons per megawatt hour.

Efficient techniques to manage water: Water can be managed in most cases to minimize environmental impacts. Methods include: well drilling, reservoir stimulation and circulation, fluids produced from the reservoir, and cooling water for heat rejection.

Drilling fluids and cuttings may be used when installing, exploring, or developing steam production and reinjection wells in geothermal power plants. Drilling fluids are water- or oil-based and may contain chemical additives that assist in the installation of steam production and reinjection wells. However, they may have negative impacts on the environment. There are several recommendations for the management of drill cuttings and fluids. They include: recovery and storage of oil-based drilling fluids and cuttings in dedicated storage tanks, lined with strong membrane-like materials prior to treatment, recycling, and/or final treatment and disposal; permanent closure of tanks to avoid the present or future release of oil-related materials into soil and water resources; reuse of drilling fluid, when possible; and disposal of water-based drilling fluids and cuttings in appropriate landfill-designated areas.

Health benefits of hot mineral springs: Hot mineral springs show various health benefits. Heat dilates blood vessels, which helps aid circulation and overall relaxation. Buoyancy takes pressure off of the joints, as it reduces the effects of gravity. And resistance helps increase muscle strength and endurance. Minerals in the water have several health benefits. Sulfur, calcium, magnesium, and lithium may have a therapeutic effect on various conditions, including: psoriasis, dermatitis, eczema, respiratory infections, arthritis, kidney diseases, and diabetes. There are governmental regulations that must be followed, depending on the country, regarding temperature, hygiene, water quality (including acidity and hardness), and safety measures (e.g., monitoring gases).

THEORY/EVIDENCE

General: Geothermal power is renewable, cost-effective, environmentally friendly, and reliable, despite being limited to geographical areas that have tectonic plate boundaries. Some examples of areas where geothermal power is available are: Japan, Iceland, Russia, the Czech Republic, and the United States.

Environment:
Direct use and heating applications have almost no negative environmental impact. Geothermal power plants
release less than one percent of the carbon dioxide emissions of a fossil fuel plant and use scrubber systems to rid the environment of hydrogen sulfide, which naturally exists in hot water and steam.

Economy: The cost of the land needed to build a geothermal power plant is typically less expensive than the cost to create nuclear, coal, gas, or oil plants. Geothermal power plants may receive government tax rebates, as this form of energy is clean and uses no fuel except the power to run the water pumps, which is self-generated. As of 2008, geothermal power has been reported to supply less than one percent of the energy in the global community. Less than half of one percent of electrical power is provided by geothermal sources in the United States. More than 25% of electrical power comes from geothermal power in Iceland; the Philippines is not far behind. Indonesia, New Zealand, Mexico, and Italy also use geothermal power.

Future of geothermal energy: Although geothermal energy is efficient and economical, the initial expense and the amount of land required to produce it deter many nations from using it.

HEALTH IMPACT/SAFETY

General: Geothermal power plants create virtually no pollutants because they do not burn fossil fuel. Visible plumes seen rising into the atmosphere from some geothermal plants are actually water vapor (steam) formations, and not smoke stacks. At times, however, harmful gases may be emitted, including carbon dioxide, nitrogen oxide, sulfur dioxide, and particulate matter. Although geothermal power plants produce some emissions, when compared to modernized coal plant emissions, they still produce far less. Recent studies showed modernized coal plants emit 24 times more carbon dioxide, 3,865 times more nitrous oxides, and 10,837 times more sulfur dioxide per megawatt hour than a geothermal steam plant.

Nitrogen oxide: Smog formation produced by nitrogen oxide may result in lung irritation and respiratory problems. Water quality may also be affected by nitrogen oxide, and acid formation can occur when nitrogen oxide reacts with sulfur dioxide. The acid that forms may fall to the earth as rain, snow, fog, or dry particles.

Particulate matter: Particles may increase the risk for asthma, bronchitis, and certain cancers.

Sulfur dioxide: Nitrogen oxides react with other substances in the air to form acids, which fall to the earth as rain, snow, fog, or dry particles. This can result in damage to forests and crops. Damage to lakes and streams may threaten wildlife. In addition, respiratory health may be affected, resulting in wheezing, asthma, and other breathing problems.
While geothermal power plants do not emit sulfur dioxide directly, once hydrogen sulfide is released as a gas into the atmosphere, it eventually changes into sulfur dioxide (and sulfuric acid).

Hydrogen sulfide: Hydrogen sulfide is released primarily as a gas and spreads into the atmosphere. However, it may be released as a liquid waste of an industrial facility or as the result of a natural event. When hydrogen sulfide is released as a gas, it remains in the atmosphere for an average of 18 hours. During this time, hydrogen sulfide may become sulfur dioxide and sulfuric acid. Exposure to low concentrations of hydrogen sulfide may irritate the eyes, nose, or throat, and may exacerbate asthma. Brief exposures to high concentrations of this chemical may cause a loss of consciousness. Usually, if a person loses consciousness and then regains it, he or she may not experience any other side effects. However, some people may experience long-term or permanent effects, including headaches, poor attention span, poor motor function, and memory loss. In serious cases, exposure to hydrogen sulfide has resulted in death. Reports of death have been found in a wide variety of different work settings, including well drilling sites, sewers, animal processing plants, waste dumps, sludge plants, and tanks and cesspools.

Mercury: Excessive exposure may cause allergic reactions, permanently damage brain and kidneys, and impair neurological development in children, even before they are born.

Other hazards
:

General: Hot mineral springs are closely monitored by laws in order to ensure the safety of those using them for health benefits. Safety measures include monitoring gas emissions, temperature, and mineral density.

Noise pollution: Noise pollution is mainly related to well drilling, steam flashing, and venting, in addition to equipment related to pumping facilities, turbines, and temporary pipe-flushing activities. Noise is not considered to be a problem, with noise levels ranging from about 80-115 decibels. This is because steps have been taken to help reduce noise, including adding sound insulation and barriers during drilling, as well as silencers, mufflers, and other soundproofing techniques on equipment in the steam processing facility. For comparison, congested urban areas typically have noise levels of about 70-85 decibels. Noise levels of cars and other sounds next to a major highway are about 90 decibels, and an airplane produces noise levels of about 120-130 decibels.

There are three main sources of noise from geothermal power plants. These include: the transformer, the powerhouse, and the cooling tower. The cooling tower produces the most noise of the three because the fans are located at the top and the tower is a relatively tall structure.

Impacts to hydrologic resources: The extraction, reinjection, and discharge of geothermal fluids may affect the quality and quantity of surface water and groundwater resources. An example of this might be a reduction in the flow of hot thermal springs due to problems when withdrawing the water. Recommended measures to prevent and control these problems include: the completion of a water balance assessment during the project planning stage to identify hydraulic connections between the geothermal extraction and reinjection points and any sources of potable water or surface water features; and isolation of steam-producing sources from shallow hydrologic formations. Physical hazards: There are physical hazards to be aware of with geothermal power plants, especially those that involve the wells and related pipeline networks. These dangers include: equipment that gives off heat at such high temperatures that anyone coming into contact with it may be immediately burned; equipment failure; or well infrastructure, which might be hazardous to anyone who might stumble on it and fall.

Recommendations to avoid hazards:
There are several ways that hazards can be avoided. These include: building fences and posting warning signs to prevent public access; installing pipelines below the surface of the earth or heat shields to prevent public contact with the high temperatures coming from geothermal pipelines; maintaining roads used to haul equipment; clean-up, disassembly and removal of equipment; and analyzing soil quality when necessary.

FUTURE RESEARCH OR APPLICATIONS

General: According to the U.S. Environmental Protection Agency (EPA), geothermal heat pumps are the most environmentally clean, energy-efficient, and cost-effective systems for temperature control. The U.S. Department of Energy (DOE) and the EPA have partnered with industry to help promote geothermal heat pumps. There are two questions facing the future of geothermal energy according to the EPA. First, can geothermal energy have a major impact on the national energy supply (being able to provide 100,000 megawatts of additional electrical capacity competitively by the year 2050)? Secondly, how much investment in research and development is needed to accomplish this?

Size of geothermal energy area: Being able to determine the heat exchange area and volume of the fractured rock in a geothermal energy source reservoir is essential to the success of the geothermal power plant. Conservative tracers, thermally and chemically reactive tracers, natural fluid tracers, and microseismic monitoring help determine the size of the resource accessed in order to help target drilling.

Improved tool development: Tools used for drilling have been designed to measure temperature, flow, and pressure on a short-term basis. They cannot, however, be left down in the wells for long periods of time. New generations of tools will need to be created that can withstand temperatures of more than 200 degrees centigrade. These tools can be used to monitor activity over the lifetime of the reservoir.

Improved model development: Although the understanding of the chemistry of rock/water systems has improved, predictive models of long-term behavior are being researched. Data are available from deep petroleum industry wells, but these data have not been collected and analyzed for their relevance pertaining to geothermal energy development.

Improved reservoir modeling: The future of geothermal energy depends mostly on the understanding of natural, unstimulated rock fracture systems and the ability of scientists to predict how the reservoir will behave when it is being drilled, fractured, or manipulated. At the same time, it is important to consider the cost of discovering geothermal energy, building ways to extract this energy, and then maintain it.

Improved water use: A better understanding of flow short circuits will allow fluids to be directed to particular parts of the reservoir. This will ultimately help avoid water loss and conserve energy.

Improved drilling patterns:
Currently, if there is a drop in temperature in extracted geothermal energy, new wells get drilled into previously unfractured rock or passages are drilled from existing wells into rock previously fractured but not accessed by circulation. Ways to create drill patterns that maximize points of drilling in rock without wasting space are being researched. Efficient solutions can take advantage of as much of the created or enhanced reservoir as possible and save money.

Geothermal energy to run batteries or cars: The topic of running cars on geothermal energy shows limited research. Results have shown that it takes more energy to create hydrogen from water than what is received in turn when burning the hydrogen in the engine.

AUTHOR INFORMATION

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

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